Fix warning about unused parameter only used in Q_ASSERT

Somehow MSVC still sees it as unused, despite our best efforts in
Q_ASSERT. So add Q_UNUSED.

 qdeclarativepinchgenerator.cpp(218) : warning C4100: 'event' : unreferenced formal parameter

Change-Id: I523b0abacd5148b2bf08fffd14b46aa8f8409f42
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>

tests/plugins/declarativetestplugin/qdeclarativepinchgenerator.cpp

@@ -218,6 +218,7 @@ void QDeclarativePinchGenerator::itemChange(ItemChange change, const ItemChangeD
 void QDeclarativePinchGenerator::timerEvent(QTimerEvent *event)
 {
     Q_ASSERT(replayTimer_ == event->timerId());
+    Q_UNUSED(event);
     Q_ASSERT(state_ == Replaying);
 
     int slaveSwipe = masterSwipe_ ^ 1;